Where you convince someone to go away with a valid reasoning (when you might be able to help them)

Usually used to save both parties time

Technical support company's might suggest a client goes to their ISP if they have internet issues to troubleshoot 1st with them, 90% of the time the issue is with the ISP however the techs could have checked a few things prior to confirm this is the case.

If something can be done but its not feasable (ie would cost millions to develop a system to do x y or z for something that's never gonna make money) advising person with request not possible is a Le Fobb